Spatial surveillance during control of infectious diseases – Salmonella Dublin in Denmark 2002-2009

Salmonella Dublin is the most commonly isolated Salmonella serotype in Danish cattle and leads to economic and welfare losses in infected herds. Furthermore, it leads to high mortality in human cases. A national surveillance program for Salmonella Dublin was initiated in Denmark in October 2002.
This study aimed at modelling the progress and spatial patterns during the control and eradication program for Salmonella Dublin, especially differences between regions and years.

A total of 27,606 cattle herds (including 7,958 dairy herds) were included in 2002. Antibody measurements of milk and blood samples were used for testing herd-level Salmonella status in dairy and non-dairy herds, respectively. Spatial clustering was analysed using the K-function and scan statistics. Geostatistics (semivariogram and kriging) was used to estimate the range of influence of Salmonella Dublin and to develop estimated risk maps.

In 2002, herd-level prevalence of Salmonella was 8% among all cattle herds (24% among dairy herds). It decreased to 4% among all cattle herds and 11% among dairy herds in 2009. Differences were seen in progress of the control among dairy herds between regions during the years. The number of clusters reduced during the study period. The range of influence between cattle herds varied between regions and in general increased during the study period.
The implementation and effectiveness of the control program was different between regions. The progress of control was seen to vary not only between regions, but also over time influencing infection dynamics. Thus, advices and regionally targeted efforts during control campaigns are needed.